Fluorescent lamps are used as a light source for reading a copy in information equipments such as facsimile, copying machine, image reader, and a display light source for a large size color display device, electronic sign board. As a conventional fluorescent lamp used for such applications, a so-called inner electrode type fluorescent lamp is known, having a lamp bulb in which a pair of electrodes are provided on both ends and a discharge medium such as rare gas etc. is enclosed. However, this kind fluorescent lamp has such drawbacks that the luminance distribution along the axis of the bulb is not uniform, that the end portions of the bulb turn black resulting in the decrease of effective light emitting length, and that the life is short. On the other hand, a so-called external electrode type dielectric barrier discharge lamp having a lamp bulb, in which a pair of external electrodes are provided on both ends is also used for the above mentioned applications. This kind of fluorescent lamp has a more uniform luminance distribution along the bulb axis by containing mercury gas than the inner electrode type fluorescent lamp, and has advantages that the bulb ends do not turn black, and the life is rather long.
Such dielectric barrier discharge lamp is operated by a lighting device shown in FIG. 1. A dielectric barrier discharge lamp 1 has such a structure that a phosphor film is formed on the inner surface of a dielectric tubular glass bulb 2 and rare gas such as neon or argon etc. and metal vapor such as mercury etc. are enclosed inside the glass bulb. Also, electrodes 3 and 4 are provided on outer surface of the both ends of the tubular glass bulb 2. These electrodes 3 and 4 are formed by winding a conductor composed of for example aluminum (Al) etc. along the circumference direction of the bulb. A high frequency power source 5 is connected between the pair of electrodes 3 and 4, with one electrode 4 being connected with the ground potential GND.
When a voltage is applied between the electrodes 3 and 4 by the power source 5, a high frequency electromagnetic field is generated between the electrodes through an electrostatic capacity between the electrodes. With the high frequency electromagnetic field, metal vapor such as mercury enclosed in the glass bulb 2 is excited to radiate the ultraviolet ray. This ultraviolet ray excites the phosphor deposited on an inner surface of the glass bulb 2 to generate a visible light, which is radiated out of the glass bulb 2.
The lighting method of the discharge lamp shown in FIG. 1 is called one side high voltage lighting method, because a high voltage of for example 2000 to 3000 V is applied on only one side of the pair of electrodes 3 and 4. With the configuration, a high voltage potential difference V is generated between the electrodes on both ends of the lamp, which is descended linearly from high voltage at the electrode 3 V to the ground potential GND at the electrode 4 as shown in FIG. 2. Such high potential difference between both ends of the lamp generates a leakage current. This leakage current is a phenomenon that a current flowing through the tubular glass bulb 2 from one electrode 3 to another electrode 4 leaks to the ground potential GND in the midway and the leakage current increases as the potential difference increases. For this reason, when a lamp is lighted by the lighting method shown in FIG. 1 and FIG. 2, the current flowing through the tubular glass bulb 2 shows current difference between the high voltage side and the GND side. Thus, according to the one side high voltage lighting method, a so-called luminances lope occurs in which the luminance of the lamp decreases from the high voltage end to the GND end of the tubular glass bulb.
Besides, there was a problem in conventional one side high voltage lighting method, that ozone is produced around the high voltage electrodes because such a high voltage for example as 2000 to 3000 V is applied to the electrode 3.
Further, in conventional one side high voltage lighting method, because the potential difference between both electrodes is great, it is confirmed that the temperature difference around the electrodes occur giving a bad effect upon the lamp action.
Therefore, it is an object of the present invention to eliminate the problems mentioned above, to prevent the luminance slope due to the leakage current based on the high voltage drive, and to supply a lighting device for dielectric barrier discharge lamp capable of obtaining almost uniform and sufficient luminance along the tube axis of the lamp bulb.